Recently, films used in video cameras are more and more often used outdoors. Further, there is a demand to make the size of the video camera small and to prolong the recordable time. Still further, there is a demand to use film to carry external memory of computers. Thus, a film having a small thickness, which has excellent durability, excellent running property and output characteristics under severe conditions, is demanded.
Magnetic recording media comprising a polyester film and a magnetic layer formed thereon by coating a ferromagnetic or metal powder are known (e.g., Japanese Laid-open Patent Application (Kokai) Nos. 61-26933 (1986) and 60-66319 (1985)). Japanese Laid-open Patent Application (Kokai) No. 62-62424 (1987) discloses a magnetic recording medium comprising an aromatic polyamide film having different Young's moduli in the longitudinal and transverse directions.
However, the above-mentioned magnetic recording media based on a polyester film or an aromatic polyamide film have drawbacks in that although there are no problems when they are used under normal temperature, their running properties and output characteristics are poor and drop out occurs when they are used at high temperatures. This is because the heat resistance of the base film is not sufficient or the surface properties and running property of the base film are not satisfactory.
An example wherein conditions of surface large protrusions of a non-magnetic material are defined is described in Japanese Laid-open Patent Application (Kokai) No. 6-195679 (1994), in which the number of large protrusions H2 on the back coat side of the film is defined as H2.ltoreq.50 protrusions/100 cm.sup.2. Japanese Laid-open Patent Application (Kokai) No. 63-146941 (1988) discloses an example wherein H2.ltoreq.100 protrusions/25 cm.sup.2 and Japanese Laid-open Patent Application (Kokai) No. 59-203232 (1984) discloses an example wherein (H3-H2).ltoreq.200 protrusions/mm.sup.2.
However, with the film disclosed in Japanese Laid-open Patent Application (Kokai) No. 6-195679 (1994), although the number of large protrusions on the back coat side of the film is defined, the number of large protrusions on the side on which the magnetic layer is to be formed is not defined, so that the magnetic medium does not attain sufficient output characteristics and running property. With the magnetic recording media disclosed in Japanese Laid-open Patent Application (Kokai) Nos. 63-146941 (1988) and 59-203232 (1984), the base film is restricted to polyester films and the number of large protrusions is large, so that they have drawbacks in that the running properties and output characteristics are poor when they are used at high temperatures.
A magnetic recording medium employing a base in which the ratio of the Young's modulus at 90.degree. C. E90 to the Young's modulus at room temperature ERT is adjusted within the range of 1.0.gtoreq.E90/ERT.gtoreq.0.6 is disclosed in Japanese Laid-open Patent Application (Kokai) No. 64-53330 (1989). However, the number of protrusions on the surface of the base is not defined, so that sufficient output characteristics as a high density magnetic recording medium cannot be obtained.
An aramide film having a dimensional change under a load of 1 kg/mm.sup.2 at 200.degree. C. for 5 minutes is not more than 5% is shown in Japanese Laid-open Patent Application (Kokai) No. 63-28695 (1988). However, this film contains 10-40% of carbon black and is thus largely different from that of the present invention.
The present inventors intensively studied the cause of these drawbacks to discover that the cause of the drawbacks resides in the heat resistance of the film and in the number of large protrusions on the surface of the other side of the base film, thereby reaching the present invention.